Torque limiters are used in aircraft flight control systems to prevent the transmission of excess torque from a drive unit when a flight control surface actuated by the drive unit becomes jammed. Flight control surfaces include, for example, a trailing edge flap on a wing.
A torque limiter commonly includes an input element coupled to an output element through a braking mechanism responsive to an over-torque condition, as may be experienced when the output element is prevented from rotation due to a malfunction. In a well-known arrangement, the braking mechanism includes an axially displaceable braking element that transmits rotation from the input element to the output element during normal operation. The braking element is spring-biased in an axial direction toward the input element, and a plurality of angularly spaced balls are received in opposing recessed pockets in the input element and the braking element. When a torque limit is exceeded, the balls roll out of the pockets and axially displace the braking element against the spring bias into frictional engagement with grounded disc brakes to frictionally brake rotation of the input and output elements.
While torque limiters of the type described above are effective in preventing damage to mechanical drive components caused by over-torque, they do not provide any protection when an over-speed condition is experienced. In the context of aircraft control systems, an over-speed condition may occur if a torque tube that transmits torque to aircraft control surfaces undergoes failure and load is suddenly removed from the output element, thereby causing the input and output elements to rotate at a dangerously high number of revolutions per minute.
What is needed is a torque limiter that is capable of responding to an over-speed condition.